容易出错的 = error prone (we do not say "mistake-prone") He is the most error-prone. This is where people make the most mistakes/errors. In English grammar the correct use of prepositions is the hardest to learn, because it is where errors are most easily made. I've tried to give you three examples of such expressions.Hope they help.

Thanks guys! All your answers are really of help.

最容易出错的 = the easiest to error in...; the most common error is...; ... is the easiest place to make mistake(s)... @@是XX中最容易出错的: During XX, @@ is the most common error. Example: During typing, hitting the wrong number is the most common error.

The grammar is most likely to go wrong in English learning .

the easiest to mistake the most error-prone make mistake the most easily most likely to go wrong（ I would like to use this one) ^_^